1. Field of the Invention
This is a method for uniquely encoding each of a plurality of swept-frequency signals such that the cross correlation function between any two signals is minimal.
2. Discussion of the Prior Art
A useful method for seismic exploration involves injection into the earth of a chirp- or swept-frequency signal that has a duration of from about six to about fifteen seconds. Classically, the swept-frequency signal is monotonic. Monotonic means that the frequency of the signal varies continuously over the duration of the sweep and no frequency is repeated. A typical frequency range might be 10-80 Hz (cycles per second), a frequency range of three octaves. An octave is a 2:1 change in frequency, e.g. 10-20 Hz. In practice, the injected swept-frequency signal propagates into the earth (land or marine environments), is reflected from subsurface earth layers and returns to the surface where the reflected signal is received by suitable strings of seismic sensors and is recorded for future processing. The recorded, reflected signal is then cross-correlated with a replica of the original swept-frequency signal to produce a seismogram. See, for example, D. L. Goupillaud, "Signal design in the vibroseis technique", Geophysics v 41, n. 6, pp 1291-1304.
David Nelson, U.S. Pat. No. 4,204,278, dated 05/20/80 teaches a quasi-periodic pulse train that is generated and swept over a repetition frequency of but a single octave. The pulsed wave train is so designed that the odd harmonics of the fundamental repetition frequency are suppressed but the even harmonics are preferentially augmented. By that method, Nelson achieves an effective swept-frequency range of several octaves from a single-octave original signal. The Nelson patent is incorporated herein by reference to the extent that it teaches a method of generating a quasi-periodic pulse train having a fundamental repetition frequency.
The result of a cross-correlation process is a cross-correlation function having a central maximum surrounded by a plurality of lesser-amplitude side lobes. It is known that the side lobes can be suppressed by randomizing the input sweep such as by adding white noise or by scrambling the sequence of the periodic signal elements. See for exmple U.S. Pat. No. 4,545,039, issued 10/01/85 to C. H. Savit, which is incorporated herein by reference, and copending patent application No. 416,582 by the same inventor, both of which are assigned to the assignee of this invention.
A secondary benefit to be derived from randomization of sweep signals, which may be likened to a unique coding scheme, is this: If two or more exploration crews are working in substantially the same geographic area and if the respective crews are using sweeps having substantially the same characteristics, then, mutual signal interference is likely to occur that may invalidate the cross-correlation process. Such interference can be minimized if each crew uses differently randomized signals that are orthogonal to one another. Orthogonality means that a cross-correlation function derived from cross-correlating the orthogonal signals is a minimum. However, although perfect orthogonality can be approached, (a zero correlation function), it can never be fully achieved.
A particularly interesting use for differently-randomized sweeps is in a two-boat marine seismic exploration operation wherein three seismic lines of profile can be surveyed for the price of only two seismic boats operating in parallel (use of three boats could provide data for five lines of profile). Each boat employs a uniquely encoded, randomized sweep for generating seismic reflection data. Boat A transmits sweep signal A which provides seismic reflection data from beneath the boat, which boat A itself receives, and seismic data from a line halfway between boats A and B, which may be received by boat B. Similarly, and concurrently with boat A, B transmits orthogonal sweep signal B which provides seismic reflection data to both boats B and A. Although each boat has recorded two overlapping sets of reflected seismic signals, those signals are easily sorted out by cross-correlation, using replicas of the A and B signals, because sweep signals A and B are orthogonal, even though they overlap in time. The same technique could, of course, be practiced on land.